Infrared imager using room temperature capacitance sensor

ABSTRACT

An infrared imager includes an array of capacitance sensors that operate at room temperature. Each infrared capacitance sensor includes a deflectable first plate which expands due to absorbed thermal radiation relative to a non-deflectable second plate. In one embodiment each infrared capacitance sensor is composed of a bi-material strip which changes the position of one plate of a sensing capacitor in response to temperature changes due to absorbed incident thermal radiation. The bi-material strip is composed of two materials with a large difference in thermal expansion coefficients.

This application is a division of Ser. No. 08/818,554 filed Mar. 14,1997, U.S. Pat. No. 5,965,886, which is a continuation of Ser. No.08/662,263 filed Mar. 27, 1996, U.S. Pat. No. 5,844,238.

FIELD OF THE INVENTION

This invention relates to room temperature capacitance sensor, and moreparticularly to a low-cost manufacturable infrared imager that operatesat room temperature and has substantially improved performanceapproaching the theoretical background limited performance limit.

BACKGROUND OF THE INVENTION

Instruments for the measurement of infrared (IR) radiation are becomingincreasingly important for a variety of commercial and non-commercialapplications. Research into the development of uncooled sensors withresponse throughout the infrared spectrum has been particularlyimportant due to the limitation on the operation of cooling systems.Uncooled infrared sensors would have important applications forspace-based remote-sensing of thermal sources, night vision, targetidentification, thermal mapping, event detection, motion detection, andothers. The limitations of the performance of the existing uncooledsensors often are the primary constraints to the performance of infraredimaging systems for many applications. As a result, there has beenconsiderable investment in the development of uncooled infrared sensors.

A broad assortment of infrared detectors has been developed over thelast 40 years. In most cases, they may be classified as either quantumor thermal detectors, depending upon whether the incoming radiation isconverted to excitations which are collected, or is converted to heatand detected through changes in temperature. In general, a quantumdetector which operates at detector temperatures Td is usually superiorto a thermal detector at the same temperatures for infrared frequenciesin which hv>>k_(B)T_(d), where h is Plaiick's constant and k_(B) isBoltzmann's constant. However, for infrared frequencies in whichhv<<k_(B)T_(d). thermal detectors represent the only functionaltechnology. The operation of quantum detectors is limited by theavailability of efficient photon conversion mechanisms, while theoperation of thermal detectors is limited by the availability ofsensitive thermometers. Only thermal infrared sensors operate in themid-to-far infrared range (λ>10 μm) at room temperature.

The pneumatic infrared detector, which was originally developed byGolay, is classified as a thermal detector. Golay's detector consists ofa small cavity filled with gas at room temperature. The cavity isseparated from the surroundings by a window and a thin, flexiblemembrane. The membrane is coated on one side with a thin metallic film,which has significant absorption throughout the infrared spectrumwhenever the sheet resistance of the film is approximately half of theimpedance of free space. The trapped gas in the Golay cell is heated bycontact with the membrane and expanded thermally, which forces themembrane to deflect outward. This deflection is usually detected withoptical or capacitive displacement transducers. At present, thesedetectors are bulky, fragile, difficult to fabricate, and expensive.Nevertheless, they have been widely used, primarily because of theirimprovement in sensitivity over all other room-temperature detectors inthe mid-to-far infrared range. Attempts to miniaturize the Golay cellfor incorporation into focal plane arrays have been unsuccessful becauseof scaling laws which relate the sensitivity of conventionaldisplacement transducers and their active area. The need for focal-planearrays of uncooled detectors stimulated the development of pyroelectricdetector arrays, the best of which are 5-10 times less sensitive thanthe Golay cell.

Current state-of-the-art uncooled IR focal plane arrays use manydifferent thermal detection mechanisms such a bolometric (sensorresistance is modulated by temperature), pyroelectric (dielectricconstant is modulated by temperature), and thermoelectric effects. Asdiscussed above, thermo-mechanical effects have been explored usingmodifications of the Golay cell. The performance of IR imagers based onthese technologies is limited compared with imagers based on directphoton conversion, such as PtSi detectors operated 77 K, and also isconsiderably worse than the theoretical background limited performance.In all approaches, the fundamental limits to the performance arecontrolled by the ability to thermally isolate the detector from itssurroundings, the detector sensitivity to a change in temperature, andthe introduction of extraneous noise sources. One of the reasons fordegraded performance is the parasitic thermal resistance paths inherentin the supporting structures of the sensing elements. Another reason isthe electronic noise present in the readout scanning circuitry.

With the above considerations in mind, the present invention is based onthe development of an IR capacitance structure that deflects theposition of a plate in response to temperature changes.

SUMMARY OF THE INVENTON

The present invention provides a high-performance infrared imager thatoperates at room temperature. More specifically, this invention uses aninfrared (IR) capacitance structure to sense changes in temperature.Thermal energy deforms the structure of the present invention resultingin a deflection that determines a capacitance which is then sensed.

The present invention provides an infrared capacitance sensor composedof a bi-material strip which changes the position of one plate of asensing capacitor in response to temperature changes due to absorbedincident thermal radiation. The physical structure of this capacitancesensor provides high thermal radiation resistance and high thermalsensitivity by utilizing a bi-material strip composed of two materialswith a large difference in thermal expansion coefficients (e.g., Si₃N₄and Al) mechanically supported by a long strip of material with highthermal resistance (e.g., Si₃N₄).

Additional embodiments within the scope of this invention are alsopossible. These embodiments are extensions of the basic IR capacitancestructure and include (1) a bridge structure with a bi-material elementfor increased structural stability, (2) a bridge structure without abi-material element, relying only on the thermal expansion and the “beambuckling concept” in which the two ends of the structure are pinned forincreased process simplicity, and (3) variations where the support armsmay be parallel or co-linear with the bi-material element.

Another aspect of this invention is the design and operation of areadout multiplexer for a focal plane imager made up of an array ofthese capacitance sensors.

Another aspect of this invention is the use of a correlated doublesampling (CDS) circuit to reduce the 1/f noise and dc offset of thepixel amplifiers.

Another aspect of this invention is the use of 2× over-sampling for boththe reference and signal samples in the CDS readout circuit so that themechanical resonant frequency of the capacitance sensor is at theNyquist frequency of the samples.

Another aspect of this invention is that the readout method does notremove the signal which is stored as a change in capacitance.

The foregoing and other aspects of the present invention will becomeapparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a physical structure of an exemplary bi-materialcapacitance sensor in accordance with the present invention.

FIG. 2 illustrates a cross-section of a pixel showing the variouselements that FIG. 1 comprises.

FIG. 3(a) is a top view of a linear array of pixels.

FIG. 3(b) is a top view of a 2-dimensional array of pixels.

FIG. 4 is a schematic circuit diagram which is useful to describe pixeladdressing.

FIG. 5(a) illustrates a stylized cross-section of the pixel of FIG. 2.

FIGS. 5(b)-5(m) illustrate the processing steps for the formation of thepixel of FIG. 2.

FIG. 6 is a geometric diagram of the exemplary device of FIG. 1.

FIG. 7 illustrates a cross-section of an excited pixel showing thedisplacement of the capacitor.

FIG. 8 is a schematic diagram of an exemplary front-end capacitancenetwork in accordance with the present invention.

FIG. 9 is a schematic diagram of an exemplary thermal circuit which isuseful for describing the thermal behavior of the structure shown inFIG. 2.

FIG. 10 is a measured amplifier noise diagram which is useful fordescribing the operation of the circuitry shown in FIG. 4.

FIG. 11 illustrates a cantilever pixel with folded support in accordancewith the present invention.

FIG. 12 illustrates a bridge style pixel with extended support inaccordance with the present invention.

FIG. 13 is a circuit diagram of an exemplary embodiment of a pixelsensor according to the present invention as incorporated into a sensorarray.

FIG. 14 is a timing diagram of the signal levels applied to theembodiment of FIG. 13.

FIG. 15 is a layout design diagram of an array of symmetric bridge stylepixels in accordance with the present invention.

FIG. 16 is an enlarged view of one of the pixels of the exemplaryembodiment of FIG. 15.

FIG. 17 is a layout design diagram of a cantilevered style pixel inaccordance with the present invention.

FIG. 18 is a circuit representation of an exemplary transmission gatefor use in the embodiment of FIG. 13.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The present invention relates to a room temperature infrared imagingsensor which can reach a performance level, known as NEΔT, in the rangeof 1 degree milliKelvin (mK), which approaches the theoretical Limit of0.36 mK. This invention, which is 100% compatible with silicon ICfoundry processing, utilizes a novel combination of surfacemicromachining and conventional integrated circuit manufacturingtechniques to produce a bi-material thermally sensitive element thatcontrols the position of a capacitive plate coupled to the input of alow noise MIOS amplifier. This combination is able to produce a roomtemperature imager with sensitivity and imagery comparable to today'sbest cooled imagers at a cost comparable to visible imagers used incamcorders. This invention achieves the high sensitivity, the low weightand the low cost necessary for equipment such as helmet mounted IRviewers and IR rifle sights.

Table 1 shows a device characteristic comparison of the currentinvention versus conventional pyroelectric and bolometric devices.

TABLE 1 Comparison Summary Sensitivity Isolation Responsivity Type (V/K)(K/W) (V/W) NEΔT (mK) Pyroelectric 0.425 5 × 10⁵ 8.5 × 10⁴   47Bolometric 0.0056 1.25 × 10⁷   7 × 10⁴ 40 Bi-material 1.08 1 × 10⁷ 1 ×10⁷ 2.6

The pixel design of the present invention has the following benefits: 1)an order of magnitude improvement in NEAT due to extremely highsensitivity and low noise, 2) low cost due to 100% silicon ICcompatibility, 3) high image quality at increased yield due to theability to do pixel-by-pixel offset and sensitivity corrections on theimager, 4) no cryogenic cooler and no high vacuum processing are needed,and 5) applicability to commercial applications such as law enforcementand transportation safety.

Infrared Capacitance Sensor

A physical structure of an exemplary bi-material capacitance sensor inaccordance with the present invention is shown in FIG. 1. The capacitorplate 100 is mechanically supported in free-space (vacuum) by abi-material strip 110 connected to a thermal isolation strip 120 whichis anchored to the substrate on one end. The length/area ratio of thethermal isolation strip 120 is large to maximize the thermal resistanceof this support. The bi-material strip 110 is composed of a bottom metallayer (e.g., aluminum) and a top layer (e.g., silicon nitride) which hasa much lower thermal coefficient of expansion than the bottom layer. Thetop plate 100 of the capacitor is covered by a thermal radiationabsorbing material 130 which causes temperature changes of the plate 100in response to incident IR radiation. The heat absorbed in the plate 100is conducted to the bi-material strip 110 by the bottom metal layerwhich has a relatively high thermal conductivity. The bi-material strip110 and the capacitor plate 100 are linearly placed in order to maximizethe displacement sensitivity.

FIGS. 2, 3(a) and 3(b) show the essential features of a single pixel ina cross-section and multiple pixels in top view pictorialrepresentations, respectively. In practice, the actual structure,positioning of the elements, and electronic addressing may varyconsiderably.

The sensor part of the pixel comprises three elements: the absorbingarea 130 formed from a photon absorbing material 210 which overlays anelectrically conductive plate 220, the area 130 converts IR radiationinto heat; the bi-material element 110 that converts heat intomechanical movement (as in a home thermostat); and the thermallyisolating support element 120 to prevent heat from being shunted down tothe substrate 200. The electrically conductive plate may, for example,be aluminum, polysilicon or indium-tin oxide.

As described above, the bi-material element 110 is composed of twolayers that differ greatly in their thermal linear expansion coefficient(described in Riethmuller, W. and Benecke, W., “Thermally ExcitedSilicon Microactuators”, IEEE Trans. Electr. Dev. v35, n6, p758, June1988, and which is hereby incorporated by reference for its teachings onthermal linear expansion coefficients). Since the two layers are bondedto one another, strain is built up and the structure bends, therebymoving the top plate of the capacitor with a sensitivity ofα=(1/C)(ΔC/ΔT)≈40%/° C. This is approximately 20 times greater than thesensitivity of vanadium oxide (having a sensitivity of approximately2%/° C.) which is currently used in the most sensitive bolometers.

The simplified circuit shown in FIG. 4 illustrates three appliedsignals, V_(A), V_(B) and V_(R) and three transistors: the resettransistor M₁, the source follower amplifier M₂, and the row selecttransistor M₃. There are additional transistors (not shown) associatedwith V_(A) and V_(B) for controlling the submicrosecond sense pulses ofopposite polarity applied to C₁ and C₂. If C₂V_(B)=−C₁V_(A) then thenominal signal applied to the amplifier transistor is zero. Thus, therelative amplitudes of V_(B) and V_(A) can be used to adjust the offsetat each pixel. Furthermore, V_(A) can be used to adjust the gain at eachpixel. Gain and offset correction at each pixel are desirable foroptimizing image quality and increasing yield.

The manufacturing techniques used to manufacture the bi-materialdetectors include surface micro-machining steps as well as silicon ICprocessing which is compatible with, and transferable to standardsilicon foundries.

FIG. 5(a) illustrates a stylized cross-section of a single pixel elementwith the uppermost layers of the IC at the bottom of the drawing. FIGS.5(b)-5(m) illustrate the processing steps for the formation of thesingle pixel element. Note that the pixel element in FIGS. 5(a)-5(m) isa single, cantilever type pixel element. The IR pixel elements aresurface micromachined above the surface of a single level metal CMOS IC.

The fabrication of a pixel begins in FIG. 5(b) with the planarization ofa metal-1 layer of an IC. The preferred planarization material 580 is aflowable oxide (FOX), approximately 800 nm thick. FIG. 5(c) shows anetch step used to define the via that will interconnect the pixelcantilever beam and the bottom plate of the sense capacitor C₁ 540.

The fabrication of the surface pixel continues in FIG. 5(d) with thedeposition and patterning of a metal-2 layer, for example, aluminum. Themetal-2 layer is patterned to form a capacitor plate 510 and vias 515.It is desirable for the metal-2 layer to be 800 nm to 1000 nm thick witha 20 nm to 50 nm Ti layer (not shown) to suppress the formation ofhillocks. In FIG. 5(e), a second planarization material 582 with aplasma etch-back is deposited to provide a uniformly planar surface forthe top level capacitor definition.

A dielectric overcoat layer 520 of silicon nitride or silicon carbideapproximately 500 nm thick is then deposited in FIG. 5(f) and patternedto act as a “stop” layer to prevent electrical contact between thecantilevered capacitor plate 220, formed in metal-3 (e.g., aluminum),and the underlying plate 510 formed in metal-2.

It is desirable that the metal-1, metal-2 and metal-3 layers beelectrically conductive. It should be noted that these layers can bealuminum, polysilicon or indium-tin oxide.

Fabrication continues in FIG. 5(g) with the deposition and patterning ofa release layer 530 for the micromachined pixel element. The releaselayer 530 is typically an oxide layer between 200 and 500 nm thick,depending on the desired trade-off between thermal sensitivity andmechanical ruggedness. The release layer 530 can also be composed ofpolysilicon. The release layer 530 functions as both a spacer layer forthe capacitor structure and the sacrificial material that will be etchedcompletely away at the end of processing. Therefore, the properties ofthe release layer 530 should be chosen to be compatible with the otherpixel structural layers.

It should be noted that the overcoat layer 520 and the release layer 530thicknesses determine the gap of the sense capacitor C₁ 540 andtherefore the thermal sensitivity. In FIG. 5(h), the overcoat layer 520and the release layer 530 are etched to form the anchor structure forthe pixel cantilever beam. This is a key process procedure since thisanchor structure desirably has not only the correct wall profile, butalso open a sufficiently large and clear opening to the metal-2 levelbelow.

Fabrication continues in FIG. 5(i) with the deposition and patterning ofthe first bi-material component. This metal-3 layer is preferably 300 nmof aluminum and forms part of the bi-material structure 110, the topplate 220 of the sense capacitor 540, and the thermal conduction layerfrom the absorption area to the bi-material element.

The next layer to be deposited and patterned is the second component 560of the bi-material structure, as shown in FIG. 5(j). This layer can besilicon carbide or silicon nitride, approximately 300 nm, and acts asthe thermal isolation element between the pixel and the substrate.

FIG. 5(k) illustrates the deposition and patterning of a very thininterconnect metal layer 570, 20 nm to 40 nm thick, which makes anelectrical connection between the via/anchor structure and the top plateof the sense capacitor 540. Platinum, titanium, titanium nitride orindium tin oxide are desirable materials for this step. This layer isdesirably thin to provide only low levels of thermal conductivity.

Note that the two layers of the bi-material structure may be reversedwith the deposition of the interconnection metal 570 placed between thenitride or carbide layer and the aluminum layer. However, thisprocessing is more difficult.

The next step, shown in FIG. 5(l), is the deposition and patterning ofthe IR absorption layer 210 at the end of the cantilever beam assembly.This material can be evaporated black platinum, carbon black, blackaluminum or other materials that have superior absorption properties.This layer may be patterned using conventional lift-off techniques. Thelayer thickness is determined by the extent to which the material“loads” the end of the pixel element.

The final step, illustrated in FIG. 5(m), is the wet chemical etching ofthe release layer 530 to free the pixel element. A conventional etchantis used to remove the release layer 530. It is desirable to use anetchant that will not significantly remove any of the layers contactingthe release layer 530. The element is then ready for bonding in asuitable package and testing.

In operation, thermal energy deforms the structure, thereby resulting ina deflection of the cantilever beam causing a change in the capacitanceof the sense capacitor. As the deflection increases, the space betweenthe plates of sense capacitor C₁ 540 increases, thereby decreasing thecapacitance of the sense capacitor 540. The reversal of this thermalinteraction, or vibration or electrostatic interaction between theplates 220 and 510 of the sense capacitor 540 may cause the top plate220 of the sense capacitor 540 to crash into the overcoat layer 520covering the lower plate 510 of the sense capacitor 540. This problem ismitigated by the addition of dimples to the structure.

Dimples are added to the structure by etching wells approximatelyone-third through the release layer 530 after the release layer 530 isdeposited and patterned in FIG. 5(g). Then, when the first bi-materialcomponent is deposited in FIG. 5(i), the first bi-material componentfills these wells, resulting in dimples on the side of the bi-materialstructure 110 contacting the release layer 530.

The vertical displacement of the center of the capacitor plate due tothe bi-material affect can be shown to vary approximately asdx/dt=0.72(α_(Al)−α_(siN))L²/t, where L is the length of the element, tis the thickness of each material, and α_(x) is the linear thermalexpansion coefficient for material x. For t=0.2 μm and L=50 μm,dx/dT=0.18 μm/° C. For a 0.4 μm gap between metal-3 and the overlay andfor a 0.4 μm overlay composed of Si₃N₄, the effective capacitor gapbetween metal-3 and metal-2 is 0.5 μm. Thus, the thermal sensitivity ofthe detection mechanism is α=(Δx/xΔT)(ΔC/CΔT)=36%/° C. The process andmaterial design rules are compatible with current silicon foundries at 1μm, which is three generations behind the state of the art. The use ofthese relaxed design rules results in low cost and high yieldmanufacturing. Analysis of the sense circuit of FIG. 8 (described indetail below) shows that the voltage response of the amplifier to atemperature change with V_(A)=10 V is dV/dT=(α/3)V_(A)≈1.2 V/° C., wherea is the thermal sensitivity. Alternative approaches such as vanadiumoxide have a voltage response that is several orders of magnitude lessthan this.

A geometric representation of the structure of FIG. 1 is shown in FIG. 6and is used for calculating the thermally induced deflection of thecapacitor plate 100. The total displacement of the capacitor plate 100is calculated as follows. For this analysis assume that the capacitorplate 100 remains planar and all of the bending occurs in thebi-material strip 110 connected to one end of the plate 100. This wouldoccur if the absorber is very thin or if it has approximately the samethermal expansion coefficient as the aluminum layer that forms the upperplate of the sense capacitor C₁. The total displacement is taken fromthe center (average) distance of the capacitor plate 100 to a referencex-axis. This displacement d equals d₁+d₂ where d₁ is the displacement atthe end of the bi-material strip 110 (given by equations (1) and (2))and d₂ is the additional displacement at the center of the capacitorplate 100 (given by equation (3)). From FIG. 6:

d ₁ =R(1−cos θ)  (1)

where θ is the angle of deflection above the horizontal, in radians;

for θ<<1, cos θ=1−θ²/2 and sin θ=θ,

d ₁ =Rθ ²/2=(where θ=L ₁ /R)L ₁/2R  (2)

d ₂=(L ₂/2)sin θ=(L ₁ L ₂)/(2R)  (3)

From an analysis of a bi-material strip (described in Shanley, F. R.,“Strength of Materials”, McGraw-Hill. 1957, p. 321, and which is herebyincorporated by reference for its teachings on bi-material stripanalysis), equation (4) is obtained:

1/R=KΔT(α₂−α₁)/t,  (4)

where:

K is a correction factor due to the different Young's moduli of the twomaterials;

ΔT is the temperature differential:

α₂−α₁ is the difference in thermal expansion coefficients of the twomaterials; and

t=t₁=t₂ is the thickness of each layer.

Substituting (4) in (2) results in equation (5):

d ₁ =L ₁ ² KΔT(α₂−α₁)/2t  (5)

For the case where L₁=L₂, the total displacement is given by equation(6):

d=d ₁ +d ₂ =L ₁ ² KΔT(α₂−α₁)/t  (6)

As a practical example, d is calculated for the following conditions:the bi-material strip is aluminum/silicon nitride, α(Al)=23×10⁻⁶,α(Si₃N₄)=2.8×10⁻⁶, L₁=L₂=50 μm, t=0.5 μm, and K=0.72. Thus, d=0.073μm/ΔT.

FIG. 7 shows a cross-section pictorial representation of a singlecantilever type pixel that is at a slightly elevated temperaturerelative to its nominal value. The bi-material effect (described inShanley and which is hereby incorporated by reference for its teachingson bi-material effects) predicts the displacement at the end of theelement due to a unit change in temperature to be KΔαL_(b) ²/2t_(b). Forthe cantilevered sensor, there is an additional displacement of thecenter of the absorber that doubles the sensitivity, e.g.,Δα/ΔT=KΔαL_(b) ²/t_(b). Table 2, which contains design parameters,summarizes the high performance and low cost dimensions of thestructure.

TABLE 2 Pixel Design Parameters High Low Symbol Definition PerformanceCost Units A Pixel area 2500 2500 μm² L Pixel length 50 50 μm t_(b)Bi-material layer thickness 0.2 0.4 μm t_(r) Release layer thickness 0.50.5 μm t_(c) Cap layer thickness 0.3 0.5 μm s Spacer 1 3 μm w_(s)Support width 2 4 μm t_(vu) Absorber-via connect thickness 250 250 ÅV_(A) Sensing voltage 10 10 Volt

The voltage response of the pixel can be analyzed through the use of theequivalent circuit shown in FIG. 8. This circuit contains a sensecapacitor C₁, a reference capacitor C₂, and the capacitance, C₃, at thegate node of the source follower amplifier (not shown). The voltage atthe gate, VG is given by equation (7):

V _(G)=(C ₁ V _(A) +C ₂ V _(B))/(C ₁ +C ₂ +C ₃)  (7)

The differential response (ΔV_(G)/ΔC₁) due to a change in the sensecapacitor is given by equation (8):

ΔV _(G) /ΔC ₁=((C ₂ +C ₃)V _(A) −C ₂ V _(B))/(C ₁ +C ₂ +C ₃)²  (8)

If C₁V_(A)=−C₂V_(B) so that V_(G)=0 when the pixel is at its nominaltemperature, then ΔV_(G)/ΔC₁=V_(A)/C_(T) where C_(T)=C₁+C₂+C₃ is thetotal capacitance at the node.

The sensor capacitance is given by equation (9):

C ₁=ε₀ A/x _(e)  (9)

where A is the pixel area and x_(e) is the equivalent thickness of thecapacitor gap, and x_(e)=t_(r)+t_(c)/ε_(c) where t_(r) is the thicknessof the release layer, and t_(c) and ε_(c) are the thickness and relativepermittivity of the cap layer (the dielectric overcoat layer 520 in FIG.5(a)), respectively. The change in capacitance induced by a displacementΔx is given by equation (10):

ΔC ₁ =−ε∘AΔx/x ² =−C ₁ Δx/x  (10)

Therefore, the voltage response to a change in temperature,R_(v)=ΔV_(G)/ΔT, at the detector is given by equation (11):

R _(v)=(ΔV _(G) /ΔC ₁)(ΔC ₁ /Δx)(Δx/ΔY)  (11)

The results are shown in Table 3.

TABLE 3 Pixel Performance Symbol Definition High Performance Low CostUnits L_(p) L-s absorber length 49 47 μm W_(p) L-4w_(s)-5s absorberwidth 37 19 μm A_(p) (L-4w_(s)-5s (L-s)) 1813 893 μm² absorber area ƒA_(p)/A fill factor 0.725 0.357 x_(c) t_(r) +t_(c)/ε_(SiN) 0.54 0.567 μmC₁ ε_(o)A_(p)/X_(c) sense 29.7 31.2 fF capacitance L_(h).L_(s) L_(p)bi-material and 49 47 μm support length Δx/ΔT KΔαL_(b) ²/t_(b) 0.1740.08 μm/K C_(T) 3C₁ total capacitance 89.1 93.6 fF R_(v) V_(A)KΔαL_(b)²/3χ_(e)t_(b) 1.078 0.472 V/K

The pixel thermal response can be analyzed through the aid of theequivalent circuit shown in FIG. 9. The radiation resistance from thescene to the detector is R_(thr). The radiation resistance between thedetector and the substrate is R′_(thr). The lumped thermal capacitance,C_(th), is proportional to the mass, specific heat, and the volume ofthe absorbing element that consists of the aluminum plate and a layer ofsilver black that has an area density of 50 μgm/cm² (described in Lang,W,. et al. “Absorbing Layers for Thermal Infrared Radiation”, Sensors &Actuators A, 34, (1992) 243-248, and which is hereby incorporated byreference for its teachings on absorbing layers). The total conductivethermal resistance to the silicon substrate is given by equation (12):

R _(c) =R _(bm) ∥R _(bi) +R _(sm) ∥R _(si)  (12)

where R_(bm) is in parallel with R_(bi) and R_(sm) is in parallel withR_(si), and R_(a), R_(bi), R_(bm), R_(si), and R_(sm) are the equivalentlumped thermal resistances of the absorber element (neglected), theinsulator and metal layers of the bi-material element, and the insulatorand metal layers of the support isolation element, respectively. Notethat the metal layer of the support element is the absorber-via metalinterconnect. The results are shown in Table 4.

TABLE 4 Pixel Thermal Performance High Symbol Definition Performance LowCost Units R_(thr) 1/4σε_(l)L_(p)W_(p)B₁T³ 1.32 × 10⁸ 1.83 × 10⁸ K/WattR′_(thr) 1/4σε_(b)L_(p)W_(p)B₁T³ 0 1.65 × 10⁹ K/Watt C_(th)(ρ_(Al)t_(s)C_(Al) + m_(Ag)C_(Ag)) 1.66 × 10⁻⁴ 1.25 × 10⁻⁴ joule/KL_(p)W_(p) R_(si) L_(s)|2K_(SiN)t_(s)w_(s) 1.22 × 10⁷ 2.94 × 10⁶ K/WattR_(sm) L_(s)|2K_(Si)t_(va)w_(s) 4.25 × 10⁷ 2.04 × 10⁷ K/Watt R_(bi)L_(b)|2K_(Si)t_(s)w_(s) 1.22 × 10⁷ 2.94 × 10⁶ K/Watt R_(bm)L_(b)|2K_(Al)t_(s)w_(s) 4.65 × 10⁵ 1.15 × 10⁵ K/Watt R_(c) 9.96 × 10⁶2.67 × 10⁶ K/Watt τ_(th) (R_(c)∥R_(thr))C_(th) 16.5 3.34 msec

In the fabrication of the surface micromachined pixels in an ICfacility, the newly developed design rules are applied to generate amask set that contains two distinct pixel structures. The surface isintegrated and pixels are fabricated in an IC facility.

More than a dozen possible pixel configurations, three of which areshown in FIGS. 11, 12 and 15, have been postulated. The variation inshading in FIGS. 11 and 12 show the finite element analysis calculatedtemperature distribution and mechanical movement due to heating at theabsorber. Note that the absorber and bi-material elements are nearly atconstant temperature. It is not possible to experimentally explore allof these structures at a reasonable time or cost. Therefore, themodeling has been performed on a few most promising structures. Forexample, the two pixel structures shown in FIGS. 11 and 12 work well.The performance of the cantilever style pixel with folded support ofFIG. 11 and bridge style pixel with extended support of FIG. 12 issummarized in Table 5. FIG. 15 shows a symmetric bridge style pixel.

TABLE 5 FEM Modeling Results Parameter Cantilever Bridge Units Spacer 11 μm Support width 3 3 μm Support thickness 0.4 0.4 μm Sensitivity 0.0720.052 μm/K Thermal resistance 1.9 × 10⁷ 1.7 × 10⁷ K/Watt Thermal timeconstant 14.9 13.5 msec 1st natural frequency 18.2 27.1 kHz 2nd naturalfrequency 99.6 60.8 kHz Spring constant 0.038 0.087 N/m Effectivedynamic mass 2.9 × 10⁻⁹ 3.0 × 10⁻⁹ grams

Other embodiments of the infrared capacitance sensor are also possible.These include (1) a bridge structure with a bi-material element forincreased structural stability, (2) a bridge structure without abi-material element, relying only on the thermal expansion of the “beambuckling concept” in which the two ends are pinned for increased processsimplicity, and (3) variations of the structure where the support armsmay be parallel or co-linear with the bi-material element. An example ofa bridge structure without a bi-material element is shown in FIG.

Infrared Imaging Array

The room temperature IR imaging array of the present invention involvesstandard IC processes. The present invention achieves NEΔT in the rangeof 1 to 10 mK even after accounting for imperfect isolation, limitedspectral bandwidth, imperfect absorption, read out transistor noise, andmechanical compromises.

A readout multiplexer for a focal plane imager is made up of an array ofthe exemplary capacitance sensors. The multiplexer may be fabricatedusing a standard single-poly double-level-metal CMOS process sequence,on which the capacitance sensor is then formed using planar depositionand etching techniques. Since the sensor is capacitively coupled to thereadout multiplexer, no direct electrical connection is required by acapacitive voltage divider circuit which includes a compensationcapacitor (with approximately the same value as the sensing capacitor)and in which the sensor and compensator plates are driven withcomplementary, bi-polar, high-voltage pulsed bias waveforms in order tomaximize the signal voltage component and cancel the dc bias components.The capacitive divider is coupled to a low-noise NIOS amplifier (e.g., asource follower) located in each pixel. Horizontal and vertical CMOSscanning registers are used to address and read out the signal from eachpixel amplifier.

The manufacturing technology required for the readout multiplexer is astandard CMOS 1-μm integrated circuit technology. Off-chip electronicshave been developed in order to demonstrate the operation of thebi-material detectors as an imaging system. The required manufacturingtechnology is standard printed circuit board technology.

Since what is being sensed is the relative position of the capacitorplates and since this is not affected by the readout method, the readoutnoise may be reduced by performing N reads on the same pixel. Thistechnique can reduce noise by a factor of N. Tradeoffs exist, however,between resolution, field size and SNR. For example, a small field ofthe wide field of view used for tracking targets may be read multipletimes to increase the local SNR.

The read out circuit is a CMOS device that is integrated on a siliconsubstrate supporting the micromachined IR detector pixels. FIG. 13 showsa schematic diagram of the sensing capacitor Cl and the readout circuitand FIG. 14 shows an illustrative timing diagram. A current mirrorconsisting of PMOS devices 740 and 742 provides the pixel load currentof 0.4 mA for the PMOS source follower 752 located in the selectedpixel. This current may be set via an external resistor 746 which has anominal value of 40 k Ω. In the IR pixel, C1 represents the thermallysensitive variable capacitance 760. Tile top plate of capacitor 760 isconnected to the readout circuit by a thin film metal resistor 764. Theother components shown in FIG. 13 are integrated in the siliconsubstrate. A column driver circuit is used to multiplex the commonsignals V_(A) and V_(B) onto one column. This circuit generates thesignals COL_VA(X) 770 and COL_VB(X) 772 under the control of the outputCOL_SELECT(X) 730 which is derived from a conventional horizontalscanning CMOS shift register (not shown). The ROW_SELECT(Y) line 710 isgenerated by a conventional vertical scanning CMOS shift register (notshown). This signal is inverted to generate the ROW_SEL_N(Y) signal 700so that the pixels in unselected rows are always in a state with thesense node VG 775 clamped to a reset potential VR 776 which is close toground. The COL_READ(X) line 720 is a vertical signal line which ismultiplexed to a common signal bus via an NMOS transistor 780 and isfurther buffered by a PMOS source follower 744 to provide an analogsignal output SIGOUTT. The output SIGOUT is provided to a CDS circuit800 which includes a clamp circuit 802, an op-amp voltage follower 804,a sample-and-hold circuit 806, and an op-amp voltage follower 808.

The readout architecture allows the imager array to easily bepartitioned into N multiple vertical sections with separate outputs,thereby making it possible to make design tradeoffs between improvedreadout noise level and more signal outputs. To describe the detailedoperation of the readout multiplexer. assume an imager of 320×244 formatoperating at 30FPS having two 1.25 MHz output ports derived fromalternating columns.

In nonselected rows of the imager array, the ROW_SELECT(Y) line 710 islow (e.g., 0 V) and the ROW_SEL_N(Y) line 700 is high (e.g.,+5 V). Thisturns on the NMOS reset transistor 750 which clamps the common sensingnode V_(G) 775 between capacitors 760 and 762 to the reset potentialV_(R) 776. Transistor 750 is a minimum geometry device (e.g., W=1 μm andL=1 μm) which has a source-drain channel resistance of approximately 5 kΩ under these biased conditions. The ROW_SELECT(Y) line 710 is connectedto the gates of the NIOS transistors 754 and 756 and turns both of thesedevices off. Transistor 754 is used to isolate the COL_READ(X) signalline 720 from the pixel PMOS source follower device 752. Transistor 756is used to isolate the top plate of capacitor 760 from the COL_VA(X)line 770.

The readout of signals from two pixels (one pixel is shown in FIG. 13)in a selected row is described below with respect to FIG. 14. At thebeginning of the readout period, t₁, the column select line 730,COL_SELECT(1), of the first pixel, switches to a high level whichconnects the signal bus and load current source 742 to the column readline 720, COL_READ(1), of the first pixel, as well as to the pixel PMOSsource follower 744. Tile row select line 710, ROW_SELECT(1), remainshigh and the inverted row select line 700, ROW_SEL_N(1), remains low foran entire line time during which all the pixels in a row are read out.The signal output at t₁ represents the reference level (SIGOUT=V_(ref))which is used during the clamp period by the external CDS processor 800.The next operation is to pulse the signals V_(A) and V_(B) for the firstpixel; namely, at time t₂, COL_VA(1) switches from low to high andCOL_VB(1) switches from high to low. The transitions have relativelyslow rise and fall times (about 50 ns) to limit the transient chargedisplacement current in the reference capacitors 762 located inunselected pixels on the same column. The complementary nature of theV_(A) and V_(B) pulses cancels the first order clock transient coupledonto the sensing node while providing a signal component proportional tothe variation of the detector capacitor 760 of the formdV_(out)=dC1*(V_(A)/CT) where CT is the total node capacitance on thesensing node. After a settling time of about 200 ns, the signal issampled at time t₃ (SIGOUT=V_(ref)+V_(p1)) by the external CDS circuit800 and the V_(A) and V_(B) pulses return to their respective originallevels. The width of the V_(A) and V_(B) pulses is minimized in order tolimit the mechanical motion induced in the detector capacitor top platedue to electrostatic attractive forces present during the sensing pulse.

After the signal for the first pixel has been sampled and processed bythe CDS circuit 800, the signal for the second pixel is read out. Attime t₄, the column select line 730 of the first pixel, COL_SELECT(1),switches low and the column select line 730 of the second pixel,COL_SELECT(2), switches high. At time t₅, the signals V_(A) and V_(B)are pulsed for the second pixel; namely, COL_VA(2) switches from low tohigh and COL_VB(2) switches from high to low. After a settling time ofabout 200 ns, the signal of the second pixel is sampled at time to(SIGOUT=V_(ref)+V_(p2)) by the external CDS circuit 800 and the V_(A)and V_(B) pulses return to their respective original levels.

The operation of the CDS circuit 800 with respect to the first pixel inthe row is now described. Transmission gates 814 and 816 are representedby switches in FIG. 13. A circuit representation of an exemplarytransmission gate 810 for use in the present invention is shown in FIG.18.

The switching cycles of clocks SW1 and SW2 which control transmissiongates 814 and 816, respectively, and the magnitude of the CDS input(SIGOUT) and output signals during the clock cycles are shown in FIG.14. An input signal of V_(ref) is held by capacitor C_(A) to establish aclamp level in the clamp circuit 802 when SW1 closes transmission gate814 at time t_(c). The stored signal remains at this level after SW1opens transmission gate 814 at time t₂. After SW1 opens transmissiongate 814, and SW2 closes transmission gate 816 at time t_(s), the heldinput signal, V_(ref), is subtracted from the current input signal,V_(ref)+V_(p1), to give an output signal of V_(p1). This output signalis held in the sample-and-hold circuit 806 and, so, remains V_(p1) afterSW2 opens transmission ate 816 at time t₃. This signal does not changeuntil SW2 closes transmission gate 816 again. A similar analysis can beperformed for the second pixel in the row.

A preliminary analysis of the readout noise sources in this designindicates that the predominant noise source is the thermal and trappingnoise associated with the pixel source follower transistor 752. The kTCnoise generated by resetting the sense node capacitance as well as the1/f noise in the pixel source follower can be suppressed by correlateddouble sampling. The thermal noise venerated in the transistor switches754 and 780 is negligible compared to the pixel source follower becausethese devices are operating in their linear region as switches with lowVds levels and have a much lower channel resistance. Using aconservative value for the CDS clamp and sample-and-hold time constantsof 25 ns, the noise bandwidth of the CDS circuit is about 6.4 MHz. Themeasured noise level of the PMOS transistor used in the design of aSamoff 640×480 element PtSi imager which is similar in size to theproposed pixel source follower is shown in FIG. 10. At frequencies abovethe 1/f knee of 0.1 MHz, the equivalent input gate noise for the PMOStransistor is about 10 nV/Hz. Integrating this value over a noisebandwidth of 6.4 MHz results in a total estimated noise voltage of 25μV−rms. The results of the NEΔT calculation is given in Table 6.

TABLE 6 Thermal Performance High Low Symbol Definition Performance CostUnits ε_(b) Bottom emissivity 0 0.1 ε_(t) Top emissivity 1 0.9 Å Upperwavelength 30 14 μm B_(λ) Spectral bandwidth factor 1.323 2.649 1 +ε_(b)/ε_(t) Bottom radiation effect 1 1.11 1 + R_(thr)/R_(c) Thermalshunting due to 13.2 68.5 support element β Thermal transfer function70.1 806 ΔT_(rms) Photon shot noise 31.6 27.3 μK NEΔT Thermal 1.91 22 mKNEΔT Amplifier 1.75 20.1 mK NEΔT Total 2.59 29.8 mK

Shot noise is common to all IR imagers and limits the ultimatecapability of an uncooled IR imager. Table 7 shows the specification ofan ideal imager which has perfect thermal isolation from the supportingsubstrate. Table 8 shows the ultimate performance based on photon shotnoise.

TABLE 7 Ideal Imager Specifications Quantity Value Units Frame rate 301/sec Thermal time constant, τ_(th) 10 msec Absorber top sideemissivity, ε 1 Absorber bottom side emissivity 0 IR spectral band 1 to100 μm Fill factor, f 100 % Pixel area, A 2500 μm² F number, F 1

TABLE 8 Ultimate Performance Limit Quantity Formula Value UnitsRadiation thermal R_(thr) = ¹/_(4εσ∫AT) _(³) 6.53 × 10⁷ W/K resistanceThermal Capacitance C_(th) = ^(τ) ^(_(th)) /_(R) _(thr) 1.53 × 10⁻¹⁰ J/KPhoton shot noise$\sqrt{\overset{\_}{{\Delta T}_{n}^{2}}} = {T_{d}{\sqrt{k}/C_{th}}}$

9.09 × 10⁻⁵ K Optical-thermal transfer β = ^(dT) ^(_(s)) /_(dT) _(d) =4F²B_(λ) 4 coefficient NEΔT${NE\Delta T}_{thermal} = {\beta \sqrt{\overset{\_}{{\Delta T}_{n}^{2}}}}$

0.36 mK

T_(s) is the scene temperature, T_(d) is the detector temperature andB₈₀ is the inverse of the fractional portion of the spectral excitanceof the scene received by the absorber. All approaches to an uncooled IRimager are limited by the factors shown. However, uncooled IR imagersdiffer in the level of thermal isolation, emissivity of the absorber,B₈₀ and sensitivity of the thermal sensing mechanism. The first twomechanisms affect the optical thermal transfer coefficient, given byequation (13):

β=4F ² B _(λ)(1+^(R) ^(_(thr)) /_(R) _(th) )(1+^(ε) ^(_(b)) /_(ε) _(t))  (13)

which prefixes all of the contributors to NEΔT including that of thesensor, given by equation (14): $\begin{matrix}{{{NE\Delta}\quad T_{sensor}} = {\beta \sqrt{\overset{\_}{\Delta \quad v_{n}^{2}}}\left( \frac{V}{T} \right)^{- 1}}} & (14)\end{matrix}$

There are two ways improve imager NEΔT: 1) reduce β by increasingisolation and IR absorption, and 2) increase sensitivity, dV/dT. Methodsto reduce β are common to all imager approaches, but the singularadvantage of the current approach over other approaches utilizingthermistors or ferroelectrics is the sensitivity of the detectionmechanism. Here, an order of magnitude improvement is shown withouthaving to discover new materials.

A unique feature of the IR detector readout multiplexer is that gain andoffset variations in the signal read from each pixel may be compensateddirectly on the imager by modulating the amplitude of the V_(A) andV_(B) pulses applied to the pixels. The modulation may be implemented bydriving the high level of V_(A) and the low level of V_(B) with theoutput of two D/A converters. The digital input data for the D/Aconverters is obtained from REM or ROM stored offset values for eachpixel. Therefore, the local nonuniformity in offset can be canceled.Similarly, since the detector response is proportional to V_(A), theoverall gain can also be adjusted. These corrections are desirablebecause they allow the device to retain a good dynamic range whileachieving high sensitivity.

The signal voltage, V_(out), produced at the gate of the pixel sourcefollower may be calculated from the superposition components due toV_(A) and V_(B), as shown in equation (15):

V _(out)=(C1V _(A) +C2V _(B))/CT  (15)

where CT is the total capacitance on V_(out).

As an example of a gain offset correction, if the D/A convertermodulation on the V_(A) and V_(B) amplitude is chosen such thatdV_(B)=−dV_(A)(C1/C2), then a gain change of dV_(A)/V_(A) will resultwithout introducing any change in the offset voltage.

FIGS. 15 and 16 show a CMOS design layout at different zoom levels foran exemplary CMOS readout circuit for an array of symmetric bridge stylepixels. FIG. 17 shows a CMOS design layout for a cantilevered stylepixel which can be incorporated into an array of pixels similar to thatshown in FIG. 15.

The key to calculating NEAT is the determination of the change ofdetector temperature with respect to a change in scene temperature(described in Jensen, A. S., “Limitations to Room Temperature IR ImagingSystems”, SPIE Vol. 2020 Infrared Technology MX (1993), and which ishereby incorporated by reference for its teachings on NEΔT). Definingthe thermal transfer coefficient as β=dT_(s)/dT_(d) where T_(s) andT_(d) are the scene and detector temperatures, respectively, NEΔT forany system can be computed from the quadrature addition of individualcontributions, as shown in equation (16):

NEΔT_(k) =βN _(rms) /S _(k)  (16)

where N_(rms) is the rms value of the noise source associated withS_(K), the signal sensitivity per K at the detector. For example, thecomponent due to amplification electronics is given by equation (17):$\begin{matrix}{{{NE\Delta}\quad T_{amp}} = {\beta {\sqrt{\overset{\_}{v_{a}^{2}}}/R_{v}}}} & (17)\end{matrix}$

where $\sqrt{\overset{\_}{v_{a}^{2}}}$

is the rms value of the equivalent input noise voltage to the sourcefollower amplifier. Similarly, equation (18) gives the thermal componentof $\begin{matrix}{{{NE\Delta}\quad T_{THERMAL}} = {\beta \sqrt{\overset{\_}{\Delta \quad T_{n}^{2}}}}} & (18)\end{matrix}$

where $\sqrt{\overset{\_}{\Delta \quad T_{n}^{2}}}$

is the thermal fluctuation of photon shot noise; in this case, thesignal sensitivity coefficient is composed of several factors such asthe imaging optics, the spectral bandwidth, and the thermal shunting dueto parasitic thermal resistance to the ambient. Thus, as stated above,the thermal transfer coefficient is given by equation (13):

β=4F ² B _(λ)(1+R _(thr) /R _(c))(1+ε_(b)/ε_(t))  (13)

where 4F² is due to the imaging optics. If the absorption in the pathbetween the source and the absorber and the spectral efficiency of theabsorber are modeled as a rectangular spectral band pass function, thenthe spectral bandwidth factor is given by equation (19):

B _(λ) =σT ⁴/∫₈ ^(λ) M _(e)(λ,T)dλ  (19)

where the lower wavelength limit is set at 8 μm due to atmosphericabsorption. The upper wavelength limit will be set by the absorberspectral properties and IR lens used by the imager. BA is assumed to berelatively independent of temperature. The photon shot noise can beshown to be given by equation (20):

ΔT _(rms) ={square root over (k/C_(th))} T  (20)

(described in Hanson, C., “Uncooled Thermal Imaging at TexasInstruments”, SPIE Vol. 2020 Infrared Technology XIX (1993), and whichis hereby incorporated by reference for its teachings on shot noise).

When the capacitance is sensed by the voltage pulse of magnitude V_(A),the absorber plate is attracted to the matching electrically conductive(e.g., metal) plate buried in the substrate with a force equal toF=C₁V_(A) ²/x and begins to move towards the substrate surface. Thisbehavior can be modeled using a lumped element model as given byequation (21):

md ² x/dt ² =εAV _(A) ² /x ² −k(i _(o) −x)  (21)

where m and k are the lumped element equivalent mass and spring constantof the mechanical system. When the pulse is removed 0.1 μsec later, asignificant amount of energy has been imparted to the plate consistingof mechanical energy due to the motion of the plate and potential energydue to the elastic bending of the support. In the absence of any dampingaction, this results in the detector plate vibrating freely until thenext sense pulse. An extreme estimate of this effect can be made byignoring the nonlinearities and the spring constant. The approximatedisplacement and energy imparted during the sense pulse application aregiven by equations (22) and (23), respectively:

d=(C ₁ V _(A) ²/2mx)t _(p) ²  (22)

E=(C ₁ V _(A) ² /x)² t _(p) ²/2m  (23)

Since there is no natural mechanism to quench these vibrations, themechanical system is coupled back into the electronic system during thereset period. In this case, the electromechanical system will followequation (24): $\begin{matrix}{{\frac{}{t}\left( {{\frac{1}{2}{m\left( \frac{x}{t} \right)}^{2}} + {\frac{1}{2}{k\left( {x - x_{o}} \right)}^{2}} + {\frac{1}{2}{C\left( {V_{R} - {I_{C}R}} \right)}^{2}}} \right)} = {{- R}\quad I_{C}^{2}}} & (24)\end{matrix}$

I _(C) =d(C(V _(R) −RI _(C)))/dt  (25)

An estimate of the behavior described is shown in Table 9. The residualresistance of the reset transistor and the series resistance in thesupport element combine to dissipate the stored energy.

TABLE 9 Mechanical Behavior High Symbol Definition Performance Low CostUnits m (ρ_(Al)t_(s) + m_(Ag))A_(p) 1.89 × 10⁻⁹ 1.41 × 10⁻⁹ gm t_(p)sense pulse width 0.1 0.2 μsec d detector plate displacement 0.015 0.061μm E energy imparted 8.0 × 10⁻¹⁴ 2.64 × 10⁻¹³ joule V_(R) reset voltage0.1 0.1 Volt R reset circuit resistance 1000 1000 Ω V_(R) ²/R initialpower dissipation 10⁻⁵ 10⁻⁵ Watt

Higher aspect ratios and further improvement in NEΔT can be achieved ifthe usual 4:3 display characteristic is ignored. For example, a 2:1aspect ratio pixel with the 2500 μm² area has sides 70.7 μm and 35.3 μmand will yield a reduction in NEΔT by a factor of 2.83.

Although illustrated and described herein with reference to certainspecific embodiments, the present invention is nevertheless not intendedto be limited to the details shown. Rather, various modifications may bemade in the details within the scope and range of equivalents of theclaims and without departing from the spirit of the invention.

What is claimed:
 1. A method of detecting radiation, comprising thesteps of: providing a microcantilever within a substrate, including thesteps of: providing a base for coupling said microcantilever to saidsubstrate and providing a thermal isolation element for isolating saidmicrocantilever from said substrate wherein the microcantilever has atleast one physical property that is affected by radiation; exposing saidmicrocantilever to a source of radiation; monitoring a radiation-inducedchange in the at least one physical property of the microcantilever todetermine a magnitude of the change; and determining a measure of theradiation from the magnitude of the radiation-induced changes in the atleast one physical property of the microcantilever.
 2. The methodaccording to claim 1, wherein the monitoring step includes monitoringradiation-induced bending of the microcantilever.
 3. The method as inclaim 1, wherein said microcantilever includes a film made of a materialwhich interacts with the microcantilever to form a bi-material element.4. The method as in claim 3, wherein said bi-material element heats whenexposed to radiation and said change in the at least one physicalproperty comprises bending.
 5. A sensor for detecting radiation inresponse to a radiation-induced change, comprising: a microcantileverformed within a substrate and including at least one physical propertythat is affected by radiation, a base coupling said microcantilever tosaid substrate, and a thermal isolation element isolating saidmicrocantilever from said substrate; and means for determining a measureof a radiation-induced change in the at least one physical property;wherein said radiation-induced change comprises a change in the at leastone physical property of the microcantilever.
 6. The sensor as in claim5, wherein said microcantilever further includes a film made of amaterial which interacts with the microcantilever to form a bi-materialelement.
 7. The sensor as in claim 6, wherein said bi-material elementheats when exposed to radiation and said change in the at least onephysical property comprises bending.